1. Field of the Invention
The present invention relates to a composite ultrasound transducer, also known as a transducer array, with piezoelectric transducer elements therein radiating substantially in their longitudinal direction. A method for manufacturing such a composite transducer, having at least one structured piezoelectric ceramic transducer element therein, is also disclosed.
1. Description of the Prior Art
Composite ultrasound transducers, also known as transducer arrays, are used in medical diagnostics. A composite ultrasound transducer consists of many small piezoelectrically active, individual transducer elements. The dimensions of the transducer elements are selected so that they radiate substantially in the longitudinal direction, i.e., in a direction along the "thickness" dimension of the transducer elements and of the transducer array. The transducer elements are contained in a plastic matrix, so that the respective longitudinal axes of the transducer elements are parallel to each other. The thickness of the individual transducer elements (i.e., their length along the longitudinal axis) determines the thickness of the composite ultrasound transducer.
Known composite ultrasound transducers are a form of a structured transducer. Structured ultrasound transducers have many advantages compared to unstructured transducers. In a structured transducer, the division of the overall transducer into individual transducer elements results in the suppression of low-frequency cross modes of oscillation, which can result in image artifacts in the resulting ultrasound image. The structuring results in a reduction in "jamming" of the transducer elements. Consequently, the coupling factor, the piezo-module, and thus the sound intensity of the composite ultrasound transducer increases in the thickness direction, and these factors approach the higher values of a bar oscillating in the longitudinal direction. The individual transducer elements can be electrically driven in groups by correspondingly divided electrodes. By doing so, the direction or focusing of the ultrasound signal is electrically variable. This can be used to advantage in phased-array transducers or annular-array transducers.
It is known that the properties of the composite ultrasound transducer depend on the form, size and arrangement of the individual transducer elements.
A composite ultrasound transducer is disclosed in German OS 34 37 862, wherein square prisms consisting of piezoelectric ceramic are embedded as transducer elements in a regular, linear arrangement in a polymer matrix. The manufacture of such a composite ultrasound transducer is undertaken by a method known as the dice-and-fill technique. In this technique, a sintered ceramic disk is divided into prisms by cross and transverse sawing. The saw kerfs have a depth which is less than the thickness of the ceramic disk, so that an uncut ceramic base or backing remains. The kerfs are filled with plastic, and the ceramic base is then ground away.
This manufacturing technique imposes restrictions and limitations on the geometry and arrangement of the individual elements of the ultrasound transducer. The fineness which can be achieved by the kerfs is limited to the thickness of the saw blade. Thus, the manufacture of ultrasound transducers with operating frequencies larger than 7.5 Mhz is possible only in limited fashion. Due to the sawing, only straight cuts with vertical edges can be generated. Therefore, the side faces of the individual transducer elements structured by sawing are parallel. It is known, however, that parallel side faces of the transducer elements favor the development of undesired cross modes of oscillation. Since the kerfs are straight cuts which proceed across the entire ceramic disk, only regular arrangements with intolerably large distances between the individual transducer elements can be generated. These large distances result in a reduction of the portion of the ceramic surface which is active for ultrasound generation.
Moreover, the precision sawing needed in the above technique is very time-consuming. The risk of damage, such as breakage of the ceramic disk, during the sawing is particularly high in the case of fine structures, as are necessary for making composite ultrasound transducers with high operating frequencies.